Metalorganic vapor phase epitaxy has been used as a crystal growth method for nitride semiconductor lasers where the device structure is of the ridge waveguide structure type. In the conventional process of manufacturing a ridge waveguide type, after an oxide film is deposited over a p-type contact layer, part of the oxide film is made a stripe and using the oxide film as a mask, a mesa structure is made through the p-type contact layer by dry-etching a p-type cladding layer. A nitride semiconductor laser thus manufactured is reported to provide excellent properties in device reliability and optical output performance.
On the other hand, the mesa structure of the nitride semiconductor laser is perpendicular or trapezoidal where the mesa width of the p-type contact layer and the mesa width over the p-type cladding layer are generally equal to, or smaller than, the mesa width of the cladding layer in the vicinity of the active layer. Furthermore, in nitride semiconductor devices, since the acceptor level of Mg as a p-type dopant is deeper than in conventional AlGaInP lasers, the acceptor's hole activation rate is low. If the Mg doping concentration is high, defects might occur in the crystal or the device characteristics might deteriorate due to Mg diffusion into the active layer. Hence, the Mg doping concentration should be below a given upper limit and the hole density is lower than in GaAs or InP lasers. For the above reasons, in nitride semiconductor lasers, the device resistance is high and as the mesa width is decreased, the device resistance rapidly increases.
Conversely, if the mesa width is increased in order to reduce the device resistance, lateral confinement would be unsatisfactory and kinks would easily occur in current-optical power characteristics at an increased threshold current or high power. For this reason, the operating voltage for nitride semiconductor lasers is higher than that for AlGaInP lasers. FIG. 1 shows relations between mesa width and device resistance for GaN lasers and AlGaInP lasers, demonstrating that the resistance of GaN lasers sharply increases as the mesa width decreases.
One solution to this problem is an inner stripe structure type laser as disclosed in the 67th Autumn Meeting, 2006, 7th Japan Society of Applied Physics p. 361, 1p-E-8. In the process of manufacturing such an inner stripe structure type laser, after growth of amorphous AlN thin film over an active layer at low temperature, the AlN layer in the mesa portion is removed by etching and an AlGaN cladding layer and a GaN contact layer are re-grown over the mesa portion and the amorphous AlN layer. Since the bandgap of the AlN layer is larger than that of the AlGaN layer, injection current is blocked by the AlN layer and confined in the mesa portion. Also, since the refractive index of the AlN layer is smaller than that of the AlGaN layer, there is a lateral refractive index difference, which improves the optical confinement efficiency. Therefore, in this conventional technique, the widths of the cladding layer and contact layer are much larger than the mesa width, increase in device resistance can be reduced even if the mesa width is decreased. However, in this technique, due to the presence of the amorphous AlN layer which has grown over the current blocking layer at low temperature, some difficulty exists in achieving high quality crystallinity on the regrowth interface or the AlGaN cladding layer over the AlN layer.
Another conventional technique is buried type lasers as described in JP-A No. 10 (1998)-93198 and JP-A No. 2000-294883.
In the manufacturing process for the buried type laser described in JP-A No. 10 (1998)-93198, after layers up to a p-type AlGaN cladding layer are stacked, a mesa structure is formed by dry-etching the layers down to the n-type GaN layer and Zn (zinc)-doped GaN as a p-type dopant is filled on lateral sides of the mesa structure to make a high resistance buried GaN layer. A GaN contact layer doped with p-type dopant Mg (magnesium) is formed over the buried layer and the mesa. Hence the contact layer area is wide. However, the mesa structure is perpendicular or trapezoidal because it is formed by dry etching and it is difficult to reduce the resistance of a layer over the cladding layer. Besides, since Zn is an easy-to-diffuse dopant, it may diffuse into the active layer during formation of the buried layer, resulting in an increase in the resistance of the active layer. Furthermore, since the buried layer on the lateral sides of the mesa structure is a high resistance GaN layer containing no Al, the lateral current blocking effect or refractive index difference is small and thus the injection current is small or the optical confinement efficiency is low.
The laser described in JP-A No. 2000-294883, uses, as a buried layer, an undoped AlGaN layer which is equal in growth temperature to MQW. Because of the use of an AlGaN buried layer, the lateral refractive index difference can be increased, and due to low growth temperature, crystallinity of the active layer hardly deteriorates; however, since the AlGaN layer is undoped, the current blocking function for the interface and buried layer is insufficient. On the other hand, buried type semiconductor lasers include InP semiconductor lasers for optical communications which typically have a buried structure having p-type and n-type InP layers stacked alternately, or a structure having a buried semi-insulating layer doped with Fe (iron). Nevertheless, if GaN or AlGaN lasers employ p-type and n-type buried layers, it is difficult to form a high-concentration p-type layer or finely control the shape of a buried structure. Besides, if Mg or Zn is used as a dopant for p-type or high resistance layers, crystallinity might deteriorate due to diffusion of Mg or Zn into the active layer.